There are two approaches for file type icons—the official and non-offical versions. If an app chooses the official icon (E.g. latest version of Microsoft Word icon for .doc files) then the icons will not be internally consistent. If the app uses all non-official icons they all tend to look a little like knock-off brands but are internally consistent.
Another consideration, is the Adobe PDF icon the official one even though OSX and Windows have their own pdf readers?